Difficulty in replacing a FT lighting wire (no CPC)

I am encountering difficulty in replacing one FT lighting wire, no CPC. It has been run in concrete, with hessian embedded, on crap-block-coke wall, with chicken wire overlay & plaster.

Removal may damage flock lined wallpaper on the other side. It is the sole 2-core cable drop to a switch, although I notice another switch & light the other side which begs the question of how that receives a supply or return with no other cable.

The alternative is 4mm^2 6491X to the architrave backbox, right back to the CU (or a locally FTE rewired lighting circuit). No class-1 lighting device, metal backbox needs an earth.

Is this still regarded as good practice?

I notice the following document suggests doing the same...

Interesting PDF used by NICEIC ECA etc.

Thanks.

-- JSB.